Solid state or wafer scale image sensors are incorporated into low cost cameras and other devices, such as mobile cellular telephones and personal digital assistants, optical devices and medical imaging devices. It is known to provide wafer scale miniature cameras incorporating lenses produced using over-molding techniques on glass substrates, which can survive the high temperature solder reflow processes used in the manufacture of such devices. Such lenses are usually stacked together, separated by spacer members to provide an air gap between each lens.
Currently non-light transmissive surfaces of such lens arrangements, for example, the inner faces of the spacer members, are coated with an anti-reflection coating, such as a black material. While such coating reduces reflections, it does not eliminate them. Thus, these surfaces define a low reflectivity specular reflector in the system. A typical reflectance of such coated surfaces may be 4% or more, which will produce undesirable ghost images and other stray light effects in the image plane of such an imaging assembly.
Furthermore, the surfaces of the glass wafer used as substrates for the lenses may reflect light outside the lens aperture. The intensity of the reflected light will be around 4% of the incident intensity, increasing with angle of incidence, and may not be dissipated by diffuse reflections.
Currently air-cavity chip scale packages (AC-CSP) have been developed for encapsulating CMOS image sensors. A typical chip scale package includes a silicon wafer onto which the complementary metal oxide semiconductor (CMOS) sensor is built by standard semiconductor processes, a spacer member comprising a wall structure mounted onto the wafer to hold a fixed “air-cavity” between the sensor surface, and a glass package lid mounted on the spacer member. The electrical contacts to the CMOS sensor are typically made by routing, from custom connection pads on front of the silicon wafer, to pads.
It may be necessary to expansion match all parts of the assembly. The glass material of the spacer member and the package lid is near expansion matched to the silicon of the wafer. Therefore it may not be possible to apply anti-reflection coatings to the components of the assembly, partly due to the stresses that reliable high quality (low surface defect occurrence) thin-film dielectric interference coatings place on the glass wafer. These stresses result in deviations in flatness and warping of the glass which affects either the process of bonding the glass spacer member and package lid to the silicon wafer, or the stresses generated in the silicon as it holds the glass in tension. This later problem results in the silicon wafer cracking as it is back-lapped (ground down to a thickness) while attached to the package lid prior to the application of the rear layer of the assembly and singulation of the glass/silicon structure to form a final chip scale package. Therefore, there is a need to reduce unwanted internal reflections within such imaging assemblies without expensive and often unsuitable anti-reflection coatings.